Apparatus for measuring electricity



H. COOCH APPARATUS FOR MEASURING ELECTRICITY Dec., 1 925- Original Filed April 17, 1925 Patented Dec. 1, 1925.

g .UNITED STATES PATENT OFFICE.

HERBERT COOCH, QF SOUTH FABNBOROUGH, ENGLAND, ASSIGNOB T0 VICKEBS LIM- ITED, 0F WESTMINSTER,

ENGLAND, A BRITISH COMPANY.

APPARATUS :F0152 MEASUBING ELECTRICITY.

Original application illed April 17, 1925, Serial No. 23,822. Divided and this application iiled October 21, 1925. Serial No. 68,823.

T0 all whom t may concewt:

Be it known that I, HERBERT -CoocH, a subject of the King of Great Britain, residing at Royal Aircraft Establishment, South Farnborough, in the county of Hants, England, have invented certain new and useful Improvements in Apparatus for Measuring Electricity (for which I have filed an application in Great Britain, No. 27,736, dated Nov. 5, 1923), of which the following is a specification. ,7 f

This invention relates to an improved apparatus for measuring electricity and p articularly for measuring small alternating electric current and is a division of application Serial No. 23,822, filed Apr. 17, 1925. The invention primarily is intended for use in measuring the strength of current or clectromotive force induced in search coils or otherl conductors on a ship, airplane or other craft by a pulsating or alternating magnetic field determined by a leader cable or the like in the sea or on the ground in navigational systems to indicate direction, or distance from or height above the cable in bad visibility. The invention however is applicable for measuring alternating current or E. M. F i in other systems.

In such navigational systems, on the one hand, the E. M. F. induced in the search coils enclosing the pulsating or alternating magnetic field of any given strength can be increased proportionately byv increasing the frequency of the magnetic eld, but, on

" the other hand, whereas the amplitude of oscillation of a measuring device tuned to the same frequency as the applied E. M. F. is an indication of the strength lof the E. M. F., the power required to give a definite amplitude of oscillation of a measuring device is greater in a device timed to a higher frequency than in a device responding to a lower frequency.

According to the present invention, a

' small E. M. Y F. of higher frequency is measured by a device operating at a lower frequency after the higher frequency has been reduced to the lower frequency.

Further, according to the present invention in a navigational apparatus wherein a search coil or the like conductorhas alternating current of a relatively high frequency induced in it by a pulsating or alternating magnetic field, a measuring dcvice qperating at a relatively low frequency is use and a requency reducer is interposed between the search coil and the measuring `device to reduce the higher frequency to the lower frequenc The invention also comprises the association of a device operating at a lower frequency for measuring a small E. M. F. of higher frequency after reduction to the lower frequency with a device for reducing the higher fre uency E. M. F. tothe lower frequency E. h F. consisting of a vibrating -reed, tuning fork, clockwork or other periodio electric contact apparatus having a natural frequency equal to the sum of or the difference between said frequencies. o

The invention further comprises for the purpose of measuringl the strength of an i. M. F. which has ad its frequency reduced,- the use of a measuring device, such f as a mirror and a tangent scale, in which `larger linear height or other indications are scales may be associated side by side or in other convenient arrangement to enable the relative strengths of the alternating current or E. M. F .s induced in the Search coils and the relative phase or polarity of the currents or E. M. F.s in the search coils to be observed.

Ihe invention is not limited to any particular frequencies of E. M. F. to be measured or any particular frequency applied to the measuring device, but in a navigational system of the nature referred to the frequency of theE. M. F. in the search coils may be of the order of say 25 to 100 cycles per second, and the frequency applied to the measuring device may be of the order of say 2 cycles per second.

In carrying out the invention, the frequency reducer may be connected in series lllO between the search coil or other circuit `carrying the E. M. F.' to be measured and device, the frequency of the contact on the;

reducer is tuned to be equal to the frequency of the phenomenon to be measured plus or minus the natural frequency of the measuring device. The frequency of the current supplied to the measuring device is thus equal to the difference between the frequency of the E. M. F. to be measured and the frequency of the reducer contact.

Condensers may be connected on each or one side of the frequency reducer in parallel with either the measuring device or the source of the E. M. F. to be measured or with both to assist by their discharge action thc supply of E. M. F. to the measuring device. be such that the search coil and one lcondenser, and the measuring instrument and the otherY condenser form tWoelectric oscillatory circuits which are connected together intermittently by the frequency reducer.

l The measuring device may be of any type of apparatus for indicating strength of current of E. M. F. provided it has a suitably low natural period of oscillation. It may be a centre zero direct current indicator which operates at a predetermined current or E. M. F.

In applying the invention to a navigational system such as indicated above comprising two separate inclined search coils arranged at an angle tov one another and acted on by the pulsating or alternating magnetic field from a sea or ground cable,

`thefrequency reducer may be combined with a low frequency relay requiring small power to operate and controlling the indicating means, and the frequency of the current in the sea 0r ground cable can be increased with a conse uent increase in lthe E. M. F. induced in t e search coils. By utilizing higher frequencies in the sea or ground cable to give a higher induced E. M. F. in the search coils, the wei ht of the search coils carried by the aircra t as well as the current in the sea or ground cable can be reduced.

The application of the present invention enables the control relay and indicating means hitherto employed to be replaced by a low frequency direct reading device giving indications of the strength of the E. M. F. induced in the search coils. case the-indicating instrument, for example,

The capacity of the condensers may In thisl may consist of a pair of coils moving in strong uni-directional magnetic fields similar to the well known methods adopted for voltmeters but preferably wound on nonconducting formers to reduce the damping effect. The measuring device can be tuned to a low natural period of oscillation so that an appreciable amplitude of swing results when very small alternating currents of the same frequency are applied. The amplitude of swing can be calibrated to indicate the E. M. F. applied.

The amplitude of swing of the measuring device may be indicated by suitable pointers attached to the moving coils or other mov-v lapproximately inversely as the distance from the ground cable and it is an advantage to have larger indications per unit E. M. F. induced in the search coils when in a Weak field or at some distance from the cable than When the Search coils are in a stronger field near the ground cable.

In the method of recordingthe angular movement of the measuring coils by spots of light refiected from mirrors on to a scale, the larger linearmovement of the spot of light on the scale per unit E. M. I". at the minimum reading end of the scale and the small linear movement per unit E. M. F. at the maximum end of. the scale may be obtained by arranging the scale in a plane which lies tangentially to the path of the moving coil. The natural frequency of swing of the moving coils is kept low so --that only low powers are required to give an angular movement. The most suitable frequency is probably one Where the speed of movement of the spot of light or pointer movement is fast enough to give an optical illusion of a quantitative moving beam and also enough to distinguish a 18()n difference in phase or direction of movement wlien two such indications are given side by side. By distinguishing' this 180 difference in phase, indications are obtained of the relative polarity of the search coils in the alterating magnetic field.

In order to illustrate the invention more elearlyits operation will be described with reference to the accompanying diagrammatic drawings as applied to the measurement of small alternating currents or E. M. F.s in

navigational apparatus where search coils are carried by an aircraft and in which coils E. M. Ff.s are mduced due to an alternating or pulsating magnetic field produced by cur- 11ersl rents in a indicated.

ln the construction of apparatus shown, two search coils l, 2, are carried by the aircraft being symmetrically disposed about a fore-and-aft vertical plane AB and inclined to the horizontal plane CD and to each other by simultaneously recording E. M. F.s induced in the coils steering directions to follow a definite path and the height of the craft above the ground are indicated. Although the coils 1 and 2 are shown as interlinkedthis is not essential and they may be separated by any desired distance. lVhen the E. M. F.s induced in the two search coils are unequal asindicated on the instrument the aircraft is not on its definite course and a steering direction is obtained to enable the aircraft to get on to the course, and when ground cable in the manner above the E. M. F.s are equal the pilot has indica-I tion that he is on his correct course and his height above the ground is simultaneously indicated.

The numeral 3 indicates a frequency reducer consisting of four double pole reversing contacts 5, 6, two of which are connected to the coil 1 and two of which are connected to the coil 2. The contacts 5, 6 are coupled or otherwise arran ed so that they are operated simultaneously y a vibrating reed, timing 'fork,'clockwork or other apparatus having a constant predetermined frequency. The current from the coil 1 passes through the frequency reducer 3 to a measuring instrument 9, and the current from the coil 2 passes through the frequency reducer 3 to a measuring instrument 10. v

The two lialves of the apparatus shown are similar and it wiil therefore suffice if one half is described in detail.

The frequency of the E. M. i?. or current induced in the search coil l may be, for example, 33 periods per second. rlhe frequency of the oscillating contacts 5 may be, for example, 35 or 31 periods per second.

, The measuring instrument 9 consists oi a coil 11 moving in a uni-directional magnetic field in a similar manner to the well known arrangement adopted in voltmeters. The voltmeter movement used is of the usual centre` zero type with a mirror 12 attached to the spindle in the position usually occupied by the pointer.

The movement of the coil against the action of springs known manner of moving coil but in the present case the spring and the coil are adjusted to Ahave a definite predetermined natural eriod of oscillation. This period of oscillation in the present case is 2 periods per second or the diiierence between the frequency of the current induced in thc search coil l and the frequency of the mov- 11 operates in the well ing contacts of the frequency reducer 3.

' A projector 13 causes a beam of light 14 instruments,`

to be reflected from the mirror 12 on to ascale15 as shown so asv to cause a spot or line of light to appear on the left hand side of the graduated scale 15 in a position depending on the angular position of the mirror. the path of the moving coil and the projector is arranged so that the spot of light is at the top end of the scale when the coil is stationary or in its zero position.

It will be appreciated that as the moving coil oscillates about its stationary or zero position the spot of light'oscillates up and down the scale, but as when the coil is in its stationary or zero position, the spot of light is at the top or one end of the scale, only one half of the oscillation of the spot of light is visible on the scale. The distance travelled by the spot of light down the scale indicates the amplitude' of oscillation of the moving coil. The scale is set tangentially to the path of the moving coil as shown in the drawing so that a large lineaimovement per unit angular coil movement is obtained at the top end of the scale and a smaller linear movement per unit anglilar coil movement is obtained at the bottom end of the scale.

The scale is graduated as shown to indicate height above the ground cable and as the E. M. F. induced in the coil is mini- The scale is arranged tangentially to mum at maximum height and maximum at minimum height, the greatest height recorded will be at the top end of the scale or in the position where there is minimum movement on the moving coil.

if the alternating E. M. F. at per second from the search coil 1 is passed through the double pole reversing contacts 5 of the frequency reducer 3 which oscillates at a frequency of 3l periods per second, the resultant E. M. F. applied to the measuring instrument 9 has an averag l value which oscillates at the rate oi' 2 periods per second. The moving coil of the measuring instrument 9 therefore receives electrical impulses at a rate equivalent to its own natural frequency of oscillation and the amplitude of oscillation is an indication of the strength of the E. M. F. induced in the search coil. It will also bc appreciated that the average current applied to thc measuring instrument 9 is in one direction when the contacts of the frequency reducer reverse most of the negative portion of the alternating curve of the current received from the search coil, and the average current in the instrument .is reversed when the frequency reducer reverses most of the positive portion of the alternating curve of the current received from the search coil. y

If we now examine the operation of the completo apparatus shown in the drawing it will be seen that the E. M. F; induced in the Search coil 1 at a frequency of 33 pc- 33 periods riods per second causes a spot of light to oscillate on the left hand side of the graduated scale at the rate of 2 periods per second, the amplitude of its oscillation being governed by the amount of E. M. F. induced l in the search coil 1. Similarly the E. M. F.

induced in the search .coil 2 causes a -second spotof light to oscillate on the right hand side of the graduated scale at an amplitude governed by the amount of E. M. F. induced in the search coil 2. The two pairs of reversing contacts of the frequency reducer 3 operate simultaneously and the connections are arranged so that the two spots of light are in synchronisni when the two Search coils are arranged so that at any instant the magnetic field passes through one coil from the under side and through the second coil fromv the top, side. If for example the search coils are placed in a magnetic field which is horizontal and in the same plane as the papei' of the drawing, the mag netic field enters from the underside of the coil 2 and the top side of the coil 1 at any instant. If however, the search coils are placed in a magnetic iield which is horizontal but in a plane at right angles to the paper, it enters from the top side of both coils at any instant, and the'induced currents in the two coils have a relative polarity 180 different from the case where the magnetic field enters from the top side of one coil and the underside of the second coil. it will be seen that the spots of light from the two instruments will oscillate in synchronism in one case but will oscillate at the same frequency but out of synchronism by 180 apart in the other case.

The instrument is therefore capable of giving simultaneously the value of E. M. F. induced in each Search coil and also the relative polarity of the two search coils one to the other, which are of advantage and of great value in -navigational apparatus of this type.

1What I claim and desire to secure by Letters Patent of the United States is 1. Navigational apparatus comprising in combination two coils in 'which alternating E. M. F. may beinduced, said coils being symmetrically disposed with relation to a vertical plane situated fore-and-aft of the craft bearing said apparatus, said coils being also inclined to the horizontal plane and to each other, a frequency reducer connected to each of said coils and a measuring device connected to each of said frequency reducers, whereby the current induced in, the said coils is reduced in frequency and indi. cated by the measuring device. Y

2. Navigational apparatus com rising in combination two coils in which a ternating E. M. F. may be induced, said coils being symmetrically disposed with relation to a .vertical plane situated fore-and-aft of the ing also inclined to the horizontal plane and to each other, a frequency reducer connected to each of s aid coils, each of said frequency i-educers being operated at uniform angular velocity, a measuring device connected to each of said frequency reducers and providing means whereby the relative strengths and phases of the E. M. F.s induced in said.

coils may be observed. p

3. Navigational aparatus comprising in combination two coils in which alternating E. M. F. may be induced, said coils being symmetrically disposed with relation to a vertical plane situated fore-and-aft of the craft beariii f said apparatus, said coils being also inclined to the horizontal plane and to each other, a frequency ieducer connected 'to each of said coils each of said frequency i'educers being operated at uniform angular velocity, a measuring device connected to each of said frequency reducers, a tangent scale for each of said measuring devices, said tangent scales being so disposed that the relative strengths and phases of the E, M. F.s `induced in said coils may be observed.

4. Navigational apparatus comprising in combination two coils in which alternating E. M. F. may be induced, said coils being symmetrically disposed with relation to a vertical plane situated fore-andaft of the craft bearing said apparatus, said coils being also inclined to the horizontal plane and to each other, a frequency reducer connected `to each of s aid coils, eachof said frequency reducers being operated at uniform angular velocity, a measuring device connected to each of said frequency reducers, the natural` frequency of each of said frequency reducers differing from that of the E. M. F. impressed on said frequency reducer by an amount equal to the frequency of the E. M. F. delivered to the corresponding measuring device.

5. Navigational means comprising an electrical conductor external to the craft to be navigated, said conductor being charged with an electric current .having periodic variations in its strength, indicating means mounted on said craft and responsive to the variations in said electric current comprising in combination two coils in which alternating E. M. F. may be induced, said coils being symmetrically disposed about a vertical plane situated fore-and-aft of said craft, said coils being also inclined to the horizontal plane and to each other, a frequency reducer connected to each of said coils each of said frequency reducers being operated at uniform angular velocity and a measuring device connected to each of said frequency reducers.

HERBERT coocH.

imis 

